1. Field of the Invention
The present invention generally relates to electronic component packages, and more specifically, to a structure of a package for surface mounting an electronic component used for high speed signal transmission.
2. Description of the Related Art
Conventionally, a sealing package for surface mounting has been widely used as a high speed IC package in an optical communication system or a communication system having an electrooptical interface circuit for signal transmission whose transmission speed is approximately 40 Gbps.
In such a sealing package for surface mounting, an electronic component is sealed in a cavity formed by a dielectric substrate, a lid connected to a surface of the dielectric substrate, and others. The electric component is connected to an electrode formed on a surface of the dielectric substrate opposite to a surface where signal lines are formed.
FIG. 1 is a view showing a related art package having an electronic component. More specifically, FIG. 1-(a) is a plan view of a related art package substrate where the lid part is not provided on an upper surface. FIG. 1-(b) is a cross-sectional view taken along a line A-A of FIG. 1-(a) where the lid part is provided on the upper surface. See U.S. Pat. No. 5,168,344, for example.
Referring to FIG. 1, a large scale integrated circuit (LSI) is mounted on a substantially center part of the package substrate 1. A ground conductor 3 made of a metal is provided outside of and surrounding the LSI 2 along four sides of the upper surface of the package substrate 1 in a rectangular ring shape.
A wall part 4a of the lid part 4 made of the metal or a dielectric body is mounted on the ground conductor 3 so that a cavity is formed inside of the lid part 4. The LSI 2 is sealed in such a cavity.
Signal lines 5 connected to the LSI 2 are provided on the upper surface of the package substrate 1. Electrodes 6 are provided in the vicinities of edge parts of a lower surface of the package substrate 1. The signal lines 5 and the corresponding electrodes 6 are connected to each other via via-holes 7 and 8 and an intermediate signal line 9 provided in the package substrate 1.
Therefore, if the package substrate 1 is mounted on a printed wiring board (not shown in FIG. 1), the LSI 2 and the signal line pattern of the printed wiring board are connected via the electrodes 6.
However, in a structure shown in FIG. 1, since a high frequency characteristic is not considered, impedance mismatch is generated at the intermediate signal line 9 positioned below the ground conductor 3 provided on the upper surface of the package substrate 1. Such an impedance mismatch causes large transmission loss when the signal frequency increases so that the communication speed is decreased.
In addition, contact between the package substrate 1 and the printed wiring board (not shown in FIG. 1) where the package substrate is mounted limits the frequency characteristic so as to affect the maximum operating frequency.
More specifically, in a case where the package substrate 1 is made of a dielectric body such as ceramic in the structure shown in FIG. 1, inventors of the present invention recognize through simulation that a −3 dB cut-off frequency is less than approximately 35 GHz.